We have been analyzing the oncogene activation process in thymic lymphomas developed in mice after carcinogen and radiation induction. We have identified the oncogenes activated as belonging to the ras family, the N-ras in carcinogen-induced tumors, and K-ras in the thymomas caused by radiation. We proceeded to isolate the activated N-ras oncogene and to study its structure through restriction mapping in concert with transfection assays. This allowed us to approximately localize the ends of the gene. A specific fragment obtained from the clone has been used as a probe to assign the N-ras gene to mouse chromosome 3. We also have cloned a piece containing the first exon of the K-ras gene activated in a radiation-induced tumor; and using recombinant constructs with the second, third, and fourth exons of a normal human H-ras gene, we have demonstrated that the transforming activity lies in that piece. Subsequent sequence of the first exon comparing the transformant gene with the normal one isolated from brain DNA from the same mouse that developed the tumor shows only a base change of G to A that will turn a glycin into an aspartic acid in the p21 protein. Synthesis of an oligonucleotide and differential hybridization with it to the thymoma DNA has confirmed that the mutation is present in the tumor. (X)